1. Field of the Invention
The present invention relates to a combustor suitable for application to a gas turbine which is enabled to run according to a load by holding a flame inside of a flame holding cone disposed in an inner cylinder that opens at its leading end into a combustion chamber and by igniting and extinguishing a plurality of main nozzles to be activated and deactivated according to the load, thereby changing the number of active main nozzles.
2. Related Art
In the prior art, there has been used a gas turbine having a combustor which is able to run according to a load by providing a pilot nozzle arranged at the central portion of an inner cylinder that opens at its end into a combustion chamber and has a flame holding cone that opens at its leading end portion into the front of the inner cylinder and by providing a plurality of main nozzles adjacent to each other around the outer circumference of the pilot nozzle so that any or all of the main nozzles may be activated according to fluctuations in the load.
In a gas turbine to be used for generating electric power, for example, any or all of the main nozzles are deactivated in the night (this time having a lower power demand). The main nozzles in a number corresponding to a load, are ignited with a flame started and ignited in a flame holding cone in the day time (having a greater power demand) so that the gas turbine may be operated to match the load.
FIG. 6 shows a construction of a combustor to be used in such gas turbine of the prior art.
As shown in FIG. 6, a pilot nozzle 02 is arranged on the axis of an inner cylinder 01, having a cylindrical shape, which is fixed at its rear end portion on a partition 011. Around the outer circumference of the pilot nozzle 02, there are arranged generally in parallel with the pilot nozzle 02 eight main nozzles 03 which are arranged at an equal circumferential pitch and which are likewise fixed at their rear end portions on the partition 011.
In the inner cylinder 01, on the other hand, there is mounted across the inner cylinder 01 a base plate 04 which is provided with through holes 05 for admitting a fuel F fed from the pilot nozzle 02 and the main nozzles 03, an air flow A for preparing a mixture M by mixing it with the fuel F, and a cooling air flow A. The partition 011, the inner cylinder 01 and the base plate 04 construct a wind box 06 around the pilot nozzle 02 and the main nozzles 03.
In front of the pilot nozzle 02, moreover, there is disposed a flame holding cone 014 which is fixed at its rear end portion on the base plate 04 and which is extended forward from the base plate 04 in a frusto-conical shape having a diverging front. The flame holding cone 014 is open at the front of the inner cylinder 01 so that the mixture M of the fuel F having passed the through holes 05 and the air flow A may be ignited to establish a later-described premixed flame 013 therein ahead of the inner cylinder 01 and in a combustion chamber 018.
Around the individual outer circumferences of the pilot nozzle 02 and each main nozzle 03, on the other hand, there are disposed a pilot nozzle outer cylinder 09 and a main nozzle outer cylinder 010 which individually open at their leading ends to communicate with the through holes 05 of the base plate 04. They are provided, on the insides of their rear ends, with a pilot swirler 07 and a main swirler 08 for swirling the air flow A introduced into the wind box 06.
In a combustor 012 for the gas turbine of the prior art thus constructed, the fuel F, as fed forward the base plate 04 from the pilot nozzle 02, is mixed with the swirling flow of the air flow A, which is introduced from the wind box 06 and swirled by the pilot swirler 07 so that it is fed through the inside of the pilot nozzle outer cylinder 09 to the front of the base plate 04, thereby preparing the mixture M. This mixture M is ignited by an igniter (not shown) which is arranged at the back in the flame holding cone 014 so that the premixed flame 013 may be held at all times at a position extending from the inside of the inner cylinder 01 ahead of the flame holding cone 014 to the inside of the combustion chamber 018.
On the other hand, the fuel F, as fed from the main nozzles 03 to the front of the base plate 04, is mixed with the swirling flow of the air flow A, which is likewise introduced from the wind box 06 and swirled by the main swirler 08 and which is fed through the inside of the main nozzles' outer cylinder 010 to the front of the base plate 04, thereby preparing the mixture M. This mixture M is ignited with the aforementioned premixed flame 013 so that the main nozzles 03 are activated to generate a combustion gas G of higher energy to generate a driving force.
In the combustor 012 used in the gas turbine of the prior art, however, a stagnation or back flow may occur in the so-called "main swirler zone 015" between the base plate 04 and the opening of the main nozzles outer cylinder 010 opened ahead of and adjacent to the base plate 04. In this case, with the main nozzles 03 being active, the premixed flame 013, which is generated inside of the inner cylinder 01 ahead of the flame holding cone 014, may propagate to the main swirler zone 015 to ignite the mixture M which has flowed to the main swirler zone 015 around the opening of the main nozzle outer cylinder 010, thereby forming the hot premixed flame 013 just ahead of the base plate 04.
Although only one main swirler zone 015 is shown in FIG. 6 so as to avoid complexity of illustration, it may be formed around any of the openings of the main nozzles outer cylinders 010 of the main nozzles 03 in the active state.
As a result, the formation of the premixed flame ahead of the base plate 04 is especially prominent when the eight main nozzles 03 are active, that is, when the combustor 012 of the gas turbine acts in full load.
The hot premixed flame 013 to be formed as the premixed flame 013 moves forward of the base plate 04 and which is formed just ahead of the base plate 04, namely, at the so-called "main swirler zone 015", will burn the main nozzles 03 and the base plate 04 to seriously to shorten the lifetime of the combustor 012.
Next, FIG. 7 shows an example of a main nozzle of the prior art which is arranged around the outer circumference of the pilot nozzle.
In the wall face of the main nozzle 03 of FIG. 7(b) downstream of the position of the main swirler 08 there are formed a plurality of or four fuel nozzle ports 025 for preparing the mixture M by injecting the fuel F into the air flow A which has entered the main nozzle outer cylinder 010 from the wind box 06 and been swirled by the main swirler 08.
In each main nozzle 03, as shown in FIG. 7, the fuel F is injected into the main nozzle outer cylinder 010 from the fuel nozzle ports 025, which is formed in the wall face of the main nozzle 03, and is swirled by the main swirler 08 so that it is mixed to form the mixture M with the air flow A in the main nozzle outer cylinder 010. This mixture M is ignited, as it flows from the main nozzle outer cylinder 010 into the combustion chamber 018, with the premixed flame 013 formed in the pilot nozzle 07 so that the combustion gas G of high energy is generated.
In the case of the main nozzle 03 which is disposed downstream of the main swirler 08 to inject the fuel F from the fuel nozzle ports 025 formed in the wall face of the main nozzle 03, as shown in FIG. 7, however, the mixture M, as prepared in the vicinity of the exit of the main nozzle outer cylinder 010, has a tendency to get a higher concentration of the fuel F at its central portion. In order to achieve a penetration of the fuel F necessary for the fuel to be efficiently mixed with the air flow A which is swirled by the main swirler 08 to flow in the main nozzle outer cylinder 010, moreover, it is necessary to inject the fuel F at a high speed from the fuel nozzle ports 025 into the air flow A in the main nozzle outer cylinder 010. This causes a disadvantage is that the pressure for feeding the fuel F to the inside of the main nozzle 03 has to be set high.